Update!
So! I suppose when I started this I thought I knew enough about electronics for something this (relatively) simple and wouldn't have to put in the hard time to learn about custom voltage regulators and the nitty gritty of this stuff but I was wrong!
After having several issues with my power circuitry and the teensy (in regards to space mostly - I wasn't quite able to figure out the coding on that and there was always something that wasn't working, so I'm switching to GPIO as I'm not using them for anything, plus less latency I guess

) I've decided to do a... waist(?) down re-do of this GBZ, new batteries, new regulators and converters, new power and charging circuitry and a new case (as I have loaded this thing with glue... can you tell I knew what I was doing?

), but the same screen and lens, button pcb, buttons and Pi.
That is unless I break something else.
Builds:
JankyBoy V1 -
worklog
Freeplay CM3 w/ Dual Thumbsticks and L2/R2 -
showcase
Infinity Sword (Kite's Circuit Sword Lite x Thanos's Infinity Gauntlet) - coming soon
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"Romans, they go the house?!"